Makefile: allow for combining DEVELOPER=1 and CFLAGS="..."

Ever since the DEVELOPER=1 facility introduced there's been no way to
have custom CFLAGS (e.g. CFLAGS="-O0 -g -ggdb3") while still
benefiting from the set of warnings and assertions DEVELOPER=1
enables.

This is because the semantics of variables in the Makefile are such
that the user setting CFLAGS overrides anything we set, including what
we're doing in config.mak.dev[1].

So let's introduce a "DEVELOPER_CFLAGS" variable in config.mak.dev and
add it to ALL_CFLAGS. Before this the ALL_CFLAGS variable
would (basically, there's some nuance we won't go into) be set to:

    $(CPPFLAGS) [$(CFLAGS) *or* $(CFLAGS) in config.mak.dev] $(BASIC_CFLAGS) $(EXTRA_CPPFLAGS)

But will now be:

    $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS) $(BASIC_CFLAGS) $(EXTRA_CPPFLAGS)

The reason for putting DEVELOPER_CFLAGS first is to allow for
selectively overriding something DEVELOPER=1 brings in. On both GCC
and Clang later settings override earlier ones. E.g. "-Wextra
-Wno-extra" will enable no "extra" warnings, but not if those two
arguments are reversed.

Examples of things that weren't possible before, but are now:

    # Use -O0 instead of -O2 for less painful debuggng
    DEVELOPER=1 CFLAGS="-O0 -g"
    # DEVELOPER=1 plus -Wextra, but disable some of the warnings
    DEVELOPER=1 DEVOPTS="no-error extra-all" CFLAGS="-O0 -g -Wno-unused-parameter"

The reason for the patches leading up to this one re-arranged the
various *FLAGS assignments and includes is just for readability. The
Makefile supports assignments out of order, e.g.:

    $ cat Makefile
    X = $(A) $(B) $(C)
    A = A
    B = B
    include c.mak
    all:
            @echo $(X)
    $ cat c.mak
    C=C
    $ make
    A B C

So we could have gotten away with the much smaller change of changing
"CFLAGS" in config.mak.dev to "DEVELOPER_CFLAGS" and adding that to
ALL_CFLAGS earlier in the Makefile "before" the config.mak.*
includes. But I think it's more readable to use variables "after"
they're included.

1. https://www.gnu.org/software/make/manual/html_node/Overriding.html

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -479,7 +479,11 @@ all::
 #
 # Define DEVELOPER to enable more compiler warnings. Compiler version
 # and family are auto detected, but could be overridden by defining
-# COMPILER_FEATURES (see config.mak.dev)
+# COMPILER_FEATURES (see config.mak.dev). You can still set
+# CFLAGS="..." in combination with DEVELOPER enables, whether that's
+# for tweaking something unrelated (e.g. optimization level), or for
+# selectively overriding something DEVELOPER or one of the DEVOPTS
+# (see just below) brings in.
 #
 # When DEVELOPER is set, DEVOPTS can be used to control compiler
 # options.  This variable contains keywords separated by
@@ -1169,7 +1173,7 @@ ifdef DEVELOPER
 include config.mak.dev
 endif
 
-ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
+ALL_CFLAGS = $(DEVELOPER_CFLAGS) $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 
 comma := ,

config.mak.dev

@@ -1,41 +1,41 @@
 ifeq ($(filter no-error,$(DEVOPTS)),)
-CFLAGS += -Werror
+DEVELOPER_CFLAGS += -Werror
 endif
 ifneq ($(filter pedantic,$(DEVOPTS)),)
-CFLAGS += -pedantic
+DEVELOPER_CFLAGS += -pedantic
 # don't warn for each N_ use
-CFLAGS += -DUSE_PARENS_AROUND_GETTEXT_N=0
+DEVELOPER_CFLAGS += -DUSE_PARENS_AROUND_GETTEXT_N=0
 endif
-CFLAGS += -Wall
-CFLAGS += -Wdeclaration-after-statement
-CFLAGS += -Wformat-security
-CFLAGS += -Wno-format-zero-length
-CFLAGS += -Wold-style-definition
-CFLAGS += -Woverflow
-CFLAGS += -Wpointer-arith
-CFLAGS += -Wstrict-prototypes
-CFLAGS += -Wunused
-CFLAGS += -Wvla
+DEVELOPER_CFLAGS += -Wall
+DEVELOPER_CFLAGS += -Wdeclaration-after-statement
+DEVELOPER_CFLAGS += -Wformat-security
+DEVELOPER_CFLAGS += -Wno-format-zero-length
+DEVELOPER_CFLAGS += -Wold-style-definition
+DEVELOPER_CFLAGS += -Woverflow
+DEVELOPER_CFLAGS += -Wpointer-arith
+DEVELOPER_CFLAGS += -Wstrict-prototypes
+DEVELOPER_CFLAGS += -Wunused
+DEVELOPER_CFLAGS += -Wvla
 
 ifndef COMPILER_FEATURES
 COMPILER_FEATURES := $(shell ./detect-compiler $(CC))
 endif
 
 ifneq ($(filter clang4,$(COMPILER_FEATURES)),)
-CFLAGS += -Wtautological-constant-out-of-range-compare
+DEVELOPER_CFLAGS += -Wtautological-constant-out-of-range-compare
 endif
 
 ifneq ($(or $(filter gcc6,$(COMPILER_FEATURES)),$(filter clang4,$(COMPILER_FEATURES))),)
-CFLAGS += -Wextra
+DEVELOPER_CFLAGS += -Wextra
 # if a function is public, there should be a prototype and the right
 # header file should be included. If not, it should be static.
-CFLAGS += -Wmissing-prototypes
+DEVELOPER_CFLAGS += -Wmissing-prototypes
 ifeq ($(filter extra-all,$(DEVOPTS)),)
 # These are disabled because we have these all over the place.
-CFLAGS += -Wno-empty-body
-CFLAGS += -Wno-missing-field-initializers
-CFLAGS += -Wno-sign-compare
-CFLAGS += -Wno-unused-parameter
+DEVELOPER_CFLAGS += -Wno-empty-body
+DEVELOPER_CFLAGS += -Wno-missing-field-initializers
+DEVELOPER_CFLAGS += -Wno-sign-compare
+DEVELOPER_CFLAGS += -Wno-unused-parameter
 endif
 endif
 
@@ -43,6 +43,6 @@ endif
 # not worth fixing since newer compilers correctly stop complaining
 ifneq ($(filter gcc4,$(COMPILER_FEATURES)),)
 ifeq ($(filter gcc5,$(COMPILER_FEATURES)),)
-CFLAGS += -Wno-uninitialized
+DEVELOPER_CFLAGS += -Wno-uninitialized
 endif
 endif